[Haifux] Announcing a new project - fakeroot-ng

[Shachar Shemesh]

Hi all,


I would like to intrude upon your time to announce an EXTREMELY 
preliminary project called "fakeroot-ng". Project web site is at 
http://sourceforge.net/projects/fakerootng. At this point there is no 
formal release, but you can get the code from SVN and it compiles fine 
on Linux i386 (yes, it requires per-platform support). I'm hoping to 
release an actual release by the end of this week.


The project is a clean reimplementation of fakeroot[1]. For those who 
don't know it, fakeroot uses LD_PRELOAD in order to wrap some system 
calls and fool the process into thinking it is running as root. Fakeroot 
further keeps tabs on the operations it is impossible for the process to 
perform, but was told it succeeded anyways, and will emulate the results 
when applicable. So, for example, if you use fakeroot to create a 
character device (which only root can), and then run "ls -l", you will 
see your character device. In actuality, it's a standard file, but 
fakeroot fools the process to think it's a character device.


Fakeroot has one major limitation, though. It does not, and cannot, 
support a chroot jail. Fakeroot's authors refuse to wrap the "open" 
system call, due to implicit recursion that it results in, which turn 
into a infinite loop. A different implementation, called "fakechroot", 
exists that does wrap "open" and does support chroot. According to the 
fakeroot authors, this implementation is highly dependent on the precise 
implementation inside glibc. Either way, the technology still dictates 
several limitations, as listed on the fakechroot web site [2].


Fakeroot-ng, which stands for fakeroot next generation, uses a whole 
different technology to wrap the system calls. Instead of using 
LD_PRELOAD, it uses ptrace to debug the process being fooled. As a 
result, it has no problem with statically linked binaries, binaries 
written in Java, or binaries linked with a different version of glibc. 
In fact, it is written in C++ without affecting the process' dynamic 
linking structure.


Of course, with the good cometh the bad. In particular, ptrace is highly 
platform dependent in every way imaginable. Also, since we are 
manipulating the process data structures from without, we do not have 
any memory to call our own with which to work. This is particularly 
painful where supporting one syscall requires actually performing two or 
more.


For example, we want to intercept the "fchmod" function, so we can 
update our internal data about SUID and similar bits (which we don't 
want to allow in the real file). Since the primary key to our database 
is the tuple <dev,inode>, and since "fchmod" contains none, we need to 
carry out the actual fchmod, but also call "fstat" in order to know what 
the device and inode numbers are. We need to turn one system call into 
two. Unfortunately, ptrace has no mechanism for generating a system call.


Fakeroot-ng solves this problem. Furthermore, it solves this problem in 
a way that requires very little #ifdef blocks (anyone who tried to read 
the strace source code has got to appreciate this fact).


How? Invite me to lecture at one of the Linux clubs and I'll tell you. 
Better yet, download the source code and look for yourself. Even better, 
send me patches to add support for more platforms.


Here is a demo of fakeroot-ng in action:

dir$ ls -la
> total 8
> drwxr-xr-x 2 sun sun 4096 2007-12-30 17:48 .
> drwxr-xr-x 7 sun sun 4096 2007-12-30 17:48 ..
> dir$ touch file
> dir$ ls -la
> total 8
> drwxr-xr-x 2 sun sun 4096 2007-12-30 17:48 .
> drwxr-xr-x 7 sun sun 4096 2007-12-30 17:48 ..
> -rw-r--r-- 1 sun sun    0 2007-12-30 17:48 file
> dir$ ../fakeroot-ng sh
> sh-3.1# ls -la
> total 8
> drwxr-xr-x 2 sun sun 4096 2007-12-30 17:48 .
> drwxr-xr-x 7 sun sun 4096 2007-12-30 17:48 ..
> -rw-r--r-- 1 sun sun    0 2007-12-30 17:48 file
> sh-3.1# whoami
> root
> sh-3.1# id
> uid=0(root) gid=1000(sun) 
> groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),107(netdev),110(powerdev),1000(sun),1001(vboxusers)
> sh-3.1# chmod 4775 file
> sh-3.1# ls -la
> total 8
> drwxr-xr-x 2 sun sun 4096 2007-12-30 17:48 .
> drwxr-xr-x 7 sun sun 4096 2007-12-30 17:48 ..
> -rwsrwxr-x 1 sun sun    0 2007-12-30 17:48 file
> sh-3.1# exit
> exit
> dir$ ls -la
> total 8
> drwxr-xr-x 2 sun sun 4096 2007-12-30 17:48 .
> drwxr-xr-x 7 sun sun 4096 2007-12-30 17:48 ..
> -rwxrwxr-x 1 sun sun    0 2007-12-30 17:48 file

Notice how "file" has the SUID bit set when inside the fakeroot environment, but not when you exit it.

comments and suggestions welcome on the fakeroot-ng mailing list [3]


Share and enjoy
Shachar


[1] http://fakeroot.alioth.debian.org/

[2] http://fakechroot.alioth.debian.org/

[3] http://sourceforge.net/mailarchive/forum.php?forum_name=fakerootng-devel